The present invention relates to optical element units for exposure processes, in particular exposure processes using immersion techniques, and, in particular, to optical element units of microlithography systems using immersion techniques. It also relates to optical exposure apparatuses comprising such an optical element unit. Furthermore, it relates to a method of preventing intrusion of contaminants into an optical element unit as well as a method of holding an ultimate optical element of an optical element unit. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image formed on a mask, reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in one or more functionally-distinct optical element groups. These distinct optical element groups may be held by distinct optical element units.
Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical element units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system.
To increase the numerical aperture so called immersion techniques have been proposed in the context of microlithography systems. With such immersion techniques, the last optical element of the optical system located closest to the substrate is immersed in an immersion medium, usually highly purified water, which is provided in an immersion zone on the substrate. With these immersion techniques numerical apertures NA>1 may be obtained.
This immersion of the last optical element causes several problems. One problem lies within the fact that the rest of the optical system usually has to be protected against any contact with the immersion medium, fractions thereof or reaction products thereof to avoid contamination of the optical elements with such contaminants. Such contaminants would otherwise lead to an undesirable degradation of the optical performance of the optical system. Thus, usually, the contact surfaces between the last optical element and the housing of the optical element holding said last optical element are sealed to avoid any intrusion of contaminants into the interior of the housing.
Usually, this sealing of the contact surfaces is achieved by gluing the last optical element to the housing, the glue forming a tight seal of the contact surfaces. Anyway, this gluing brings along several disadvantages. One of them is the fact that, due to different coefficients of thermal expansion of the last optical element and the housing, the glue seal is subjected to varying stresses during operation. These varying stresses, of course, may have adverse effects on the useful life of the glue seal.
Furthermore, the glue itself may be a source of contaminants leading to a contamination of the optical elements and, consequently, to an undesirable degradation of the optical performance of the optical system.
A further problem lies within the deformation of the last lens element due to different coefficients of thermal expansion of the last optical element and the housing. These deformations have a considerably stronger influence on the imaging quality than in conventional “dry” exposure processes without an immersion medium. In conventional “dry” exposure processes with air or gas being present at both sides of the last optical element, compensation effects occur upon deformation of the last optical element due to the approximately identical refractory index ratios on both sides of the last optical element. Since the refractory index of the immersion medium is similar to the refractory index of the last optical element, these compensation effects do not occur. Thus, such thermally induced deformations of the last optical element are to be avoided to the greatest possible extent.
A solution would be to provide a housing that has a coefficient of thermal expansion that is comparable to the coefficient of thermal expansion of the last optical element. Anyway, such a housing would be rather expensive.
The problem of the intrusion of parts of the external atmosphere into the interior of the housing of the optical system used in the exposure process does not only exist for optical exposure systems using immersion techniques. For example, when using light in the so called vacuum ultraviolet range for a conventional “dry” exposure process, the light of this range is strongly absorbed by a large variety of substances such as oxygen, water vapor etc. Thus, it is necessary to provide a low absorption atmosphere throughout the entire path of the light to maintain the exposure performance of the system. Thus, sometimes, a low absorption gas such as nitrogen is used as the atmosphere surrounding the housing of the optical system while, e.g. for thermal reasons, a low absorption gas such as is used as the medium within the housing. Since the refractory index of nitrogen considerably deviates from the refractory index of helium, it is necessary to prevent intrusion of the nitrogen into the housing of the optical system in order to maintain stable imaging properties of the optical system within the housing.
To avoid such contamination of the inner part of the housing of the optical system it is known from EP 1 339 099 A1 to Shiraishi to provide a long and narrow axial gap between an ultimate optical element and the housing of the optical system and to draw off sub-stances entering the gap via an exhaust channel open towards the gap. The gap is provided via three conical posts protruding from the housing on which the ultimate optical element is sitting. The ultimate optical element is axially clamped between the housing and a clamping ring. The clamping ring contacts the ultimate optical element via three conical posts protruding from the clamping ring and being axially aligned with the corresponding conical posts protruding from the housing.
This solution, on the one hand, has the disadvantage that, due to the drawing off process in the gap, the pressure within the housing may drop considerably leading to an unwanted deformation of the ultimate optical elements. A further disadvantage lies within the fact that the gap width is defined by the fixed dimensions of the posts such that an adjustment of the gap width is not possible. Furthermore, again, the differences in the coefficient of thermal expansion between the ultimate optical element lead to the introduction of deformations into the ultimate optical element clamped between the housing and the clamping ring.